The present invention relates to a solvent for the chromogenic dye-precursor material for a pressure-sensitive recording paper sheet and a pressure-sensitive recording paper sheet prepared by using the solvent, and more in detail relates to a solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, which comprises (1) 30 to 80% by weight of p-monoisopropylbiphenyl or a biphenyl mixture of not less than 80% by weight of p-monoisopropylbiphenyl, not more than 20% by weight of m-monoisopropylbiphenyl and not more than 10% by weight of diisopropylbiphenyl, and (2) 70 to 20% by weight of diisopropylnaphthalene or a naphthalene mixture of not less than 97% by weight of diisopropylnaphthalene, not more than 1% by weight of monoisopropylnaphthalene and not more than 2% by weight of triisopropylnaphthalene, and the pressure-sensitive recording paper sheet prepared by using the above-mentioned solvent as a solvent for the chromogenic dye-precursor material.
A pressure-sensitive recording paper sheet comprises a colour-development sheet prepared by coating microcapsules in which a solution of a colourless electron donating chromogenic dye-precursor material having a colouring reactivity has been encapsuled, onto a supporting sheet and a colour-developer sheet prepared by coating a colour-developer which develops a colour on contacting to the chromogenic dye-precursor material onto a supporting sheet.
In recent years, the pressure-sensitive recording paper sheets have been broadly used instead of carbon copying paper sheets and back-carbon copying paper sheets of pigment type.
As the pressure-sensitive recording paper sheets, it is necessary that they are excellent in colour-development, stability for long term preservation and light resistance and that they are low in toxicity not to cause environmental pollution.
As the solvent which dissolves the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, it is demanded that the solvent fulfills the following requirements.
(1) To dissolve the chromogenic dye-precursor material to a high concentration.
(2) Not to cause the decomposition and colour-development of the chromogenic dye-precursor material.
(3) To show a considerably high boiling point, and not to evaporate in the thermal drying step and under high atmospheric temperature.
(4) Not to reduce to water on encapsulating.
(5) To show a high speed of colour-development and a high concentration of the developed colour as well as the high colour stability after colour-developing.
(6) To be stable to light, heat and chemicals.
(7) To show a low viscosity so that its flow out from the broken capsules is freely carried out.
(8) To be substantially odorless.
(9) To show a low toxicity to human body and to be safe.
(10) To show a favorable biodegradability and not to cause environmental pollution.
As the solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, which fulfills the above-mentioned requirements, several solvents have been proposed. For instance, (1) a mixed solvent comprising more than 30% by weight of isopropylbiphenyl (represented by the formula: ##STR1## less than 55% by weight of polyisopropylbiphenyl and less than 15% by weight of biphenyl, which is used for preparing a solution of the chromogenic dye-precursor material to be contained in the capsules used for coating onto a sheet material for pressure-sensitive recording paper sheets (U.S. Pat. No. 3627581), (2) a solvent for the chromogenic dye-precursor material, comprising at least one of C.sub.1-12 -alkylated biphenyl or C.sub.1-12 -alkylated terphenyl, or a mixture of the alkylated-biphenyl or -terphenyl and other solvent [wherein the number of the alkyl group in the alkylated biphenyl is 1 to 4, that in the alkylated terphenyl is 1 to 6 and not less than two alkyls may be the same or different from each other](British Patent No. 1352597) and (3) a solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, comprising a combination of 10 to 100 parts by weight, preferably 30 to 70 parts by weight of monoisopropylbiphenyl with 100 parts by weight of diisopropylnaphthalene (Japanese Patent Publication No. 50-14570 (1975)).
With the propagation of the pressure-sensitive recording paper sheets, the case wherein the pressure-sensitive recording paper sheets are used in cold districts of not more than -5.degree. C. in the ambient temperature or transported and preserved for a long time in the environment of about 40.degree. to 50.degree. C. in the ambient temperature and of higher than about 80% in relative humidity has increased.
Particularly, in the outdoor facilities such as gasoline service stands, the pressure-sensitive recording paper sheets are used under the environment of not more than -5.degree. C. in winter.
Since in such an environment of low atmospheric temperature, (1) the solvent of the chromogenic dye-precursor material used in the pressure-sensitive recording paper sheet crystallizes, (2) it is necessary for a very long time in order to clearly develop or (3) the colour-developed image is very light in colour, if developed, not to be deciphered, such a pressure-sensitive recording paper sheet is not to be put to practical use. Namely, it is demanded that an initial colour developing activity within 30 sec from the recording is at least 40%.
"Isopropylbiphenyl" disclosed in the Japanese Patent Publication No. 50-14570 (1975) and U.S. Pat. No. 3627581 as the solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet is a mixture of isomers represented by the formula: ##STR2## wherein the isopropyl group occupies the o-, m- or p-position of the benzene ring of biphenyl.
The commercialized "isopropylbiphenyl" and the isopropylbiphenyl synthesized by Friedel-Crafts alkylation of biphenyl (refer to Industrial and Engineering Chemistry Product Research and Development, Vol. 8, 239-241, 1969) is a mixture of m-isomer and p-isomer containing a small amount of o-isomer. Such a mixed solvent exhales an offensive odor strongly and is not to be used as the solvent for the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet (refer to Japanese Patent Publication No. 50-14570 (1975)).
The problem of offensive odor occurs in the process for preparing the pressure-sensitive recording paper sheet when the recording paper sheet material prepared by coating the microcapsules containing the solution of the chromogenic dye-precursor onto the supporting sheet is cut after drying. Namely, when the pressure-sensitive recording paper sheet is cut at a relatively high temperature of 40 to 50.degree. C., the solvent flowing out from the thus broken microcapsules gives a disagreeable impression to the operators. In addition, on the cases when the solvent adheres to clothes or hands in the cutting step of the pressure-sensitive recording paper sheet or in the handling of the solvent, the odor still remains even after washing the clothes or the hands with a cleanser to give a disagreeable impression. Furthermore, the pressure-sensitive recording paper sheets which have been subjected to recording are assembled and preserved in a storehouse for a relatively long time. In such occasion, the odor emitted from a large amount of the pressure-sensitive recording paper sheets which have been subjected to recording becomes to be the cause of disagreeable impression.
As has been shown above, the problem of the disagreeable odor concerning the pressure-sensitive recording paper sheets has been conspicuous in recent years.
As a result of the present inventors' studies for obtaining a solvent for the chromogenic dye-precursor material for the pressure-sensitive recording paper sheets, which is almost odorless, shows an excellent colour-development even at a low temperature of -5.degree. C., and does not crystallize at such a low temperature of -5.degree. C., it has been found that a solvent prepared by admixing diisopropylnaphthalene with p-monoisopropylbiphenyl at a specified ratio is almost odorless, does not crystallize at a low temperature of -5.degree. C. and fulfills all the above-mentioned requirements which are to be possessed by the solvent of the chromogenic dye-precursor material for the pressure-sensitive recording paper sheet, and based on the findings, the present inventors have attained the present invention.